Ways to add monitors:
Video card with two or more ports. Some cards can do only two screens even if there are more ports.
More video cards if you have space inside, a slot open on motherboard, and power for the cards from PSU
A motherboard port if it can be active as same time as a video card is plugged in. Check system's BIOS settings.
A USB external video adapter and its software. Several might be added.
A USB monitor and its software. Ditto.
A Matrox Head2Go or other type adapter box to make more screens from one cord. Best if the monitors match.
Another computer's screen, linked through network cable, and with special software for Windows or Apple Mac OS systems. Search Google how.

If you have two connectors on your computer for monitors then you can connect two monitors. These can be on two different video cards or both on the same card. If you PC has a VGA connector and a Display port connector, then you can attach one monitor to each. Or if it has a vga and a hdmi port, or any other set of two or more ports. You can use as many monitors as you have ports to connect them.